This invention relates to cementing casing in subterranean formations. In particular, this invention relates to methods for initiating circulation through the well bore to allow cement composition to flow into the well bore at low pressure. The circulation may be established in either reverse-circulation or conventional-circulation directions.
Typically, prior to cement operations, casing is inserted in a well bore. Circulation fluid fills the inner diameter (“ID”) of the casing and the casing-by-well bore annulus. For purposes of this disclosure, “circulation fluid” is defined as circulation fluid, drilling mud, and/or any other fluid typically found in precement wells. Once stagnant in the well bore, the circulation fluid has a certain gel strength that renders the circulation fluid resistive to flow initiation. Thus, a higher pumping pressure is required to initiate fluid circulation than is required once circulation is established. Further, because cement composition is typically heavier than circulation fluid, once a sufficient amount of cement composition has been pumped into the well bore, gravity will pull the cement composition down into the well bore to drive fluid circulation through the well bore.
One method of pumping cement composition into the casing-by-well bore annulus involves pumping the cement composition down the casing at the well head. The cement composition is pumped at high pressure down the ID of the casing until it reaches a casing shoe. The cement composition then exits the casing ID into the annulus through the casing shoe. The cement composition then flows up the annulus from the casing shoe. Circulation fluid is usually pumped down the casing ID behind the cement composition to drive the cement composition through the casing shoe and up the annulus. In most instances, high pressure pumps and pumping systems are required to lift the cement composition from the casing shoe in the annulus. This establishes fluid flow in a conventional-circulation direction.
Another method of pumping a cement composition into the casing-by-well bore annulus involves pumping the cement composition directly into the annulus at the well head, which is generally referred to as “reverse-circulation.” The circulation fluid flows in a reverse-circulation direction from the annulus, through the casing shoe and up through the ID of the casing where it flows out of the well head. Generally, this pumping method requires
somewhat lower pumping pressures than flowing the fluid in the conventional direction, because the weight of the cement composition in the annulus helps to drive fluid flow.
In cases where cementing operations commence after the circulation fluid in the well bore has become stagnant, the gel strength of the circulation fluid and/or drilling mud should be overcome to initiate fluid circulation through the well. In both conventional-circulation and reverse-circulation methods, a certain pump pressure should be obtained to initiate fluid circulation. Circulation should be established to allow a sufficient quantity of cement composition to flow into the well bore for the weight of the cement composition to maintain fluid circulation.